Spine003 keeps stretching book in undesirable way
If you look at it in edit mode and show the item properties, you'll see that the vertices are linked to several groups and two of them corresponds to two bones.



Depending on what you want to do, you may remove the unwanted bones influences from all the vertices of the book.

If the book is not supposed to be bent maybe you should keep only one bone influence for it (all its vertices).